Method of making fibre reinforced thermoplastics material structure

ABSTRACT

A fibre reinforced thermoplastics material structure having a first layer which is fully consolidated with uniformly dispersed fibres and a second layer which is formed as an absorbent matrix.

This application is a continuation of application Ser. No. 07/078,172,filed July 27, 1989) abandoned.

This invention relates to a fibre reinforced thermoplastics materialstructure and to a method of making such a structure, the resultingarticle having flow moulded detail on one side and an open, porousstructure on the other. This open structure can be subsequentlyimpregnated with a thermosetting resin to impart desirable features.

As described in European Patent Application No. 85300034.7 (EuropeanPublication No. 0 148 763), the subject matter disclosed in thatapplication being incorporated by reference herein, consolidatedthermoplastic material reinforced with long, stiff fibres will expandwhen heated to a temperature such that the viscosity of thethermoplastic material is sufficiently reduced to allow movement of thefibres; which occurs due to release of stresses in the fibre network.This phenomenon occurs to a varying extent depending on the type/gradeof thermoplastic, the proportion of fibre, and the type/dimensions ofthe fibre.

When such a material is heated and expanded it can be moulded to shapein two ways:

(1) Conventionally, whereby a charge of material is placed in the mould,which when closed, forces the material to flow and fill completely themould cavity. An article produced in this way is fully consolidated(densified) and can be made to contain intricate detail fully reinforcedwith glass fibre. Depending on the intended application such an articleis either ready for use or it can be coated or painted with suitablematerials. However, it cannot be impregnated due to its fully densifiedstate.

(2) A sheet of hot, expanded material is inserted to cover the lowertool of the mould. The mass of this sheet is insufficient to fill themould in a fully densified state so that when the mould is closed to thelimit of its travel a semi-consolidated form is produced having aresidual porosity. This porous nature can be used as a means whereby theproduct can be impregnated with a liquid resin if so desired. However, adisadvantage of this forming technique is that flow moulded details(such as deep ribs) cannot be formed without losing porosity in theregion of flow. Thus if uniform impregnation is required on one surfaceof the article some sacrifice in the extent of fibre reinforced mouldeddetail on the reverse side may be necessary.

The invention disclosed herein describes a means whereby it is possibleto produce an article having the attributes of both the above methods;i.e. an article having a flow moulded features, such as fibre reinforcedribs, on one side whilst retaining the ability to uniformly absorbliquid resin on the other. It has also been discovered that articlesproduced by this invention are free of sink marks, which is a problemfeature of articles produced by the method 1 above.

According to the present invention a fibre reinforced thermoplasticsmaterial structure comprises a first layer which is fully consolidatedwith uniformly dispersed fibres and a second layer which is formed as anabsorbent matrix.

Thus, the absorbent matrix can be invested with a thermosetting orthermoplastics material to achieve the result set forth above.

The invention includes a method of making a fibre reinforced plasticsstructure as set forth above which includes laminating a first sheet offibre reinforced thermoplastics material which will consolidate whencooled after subjection to pressure at a temperature above the melttemperature of the thermoplastic with a second sheet of fibre reinforcedthermoplastics material which will remain substantially unconsolidatedand porous when cooled after subjection to said temperature andpressure, and shaping the laminate in a mould at said temperature andpressure so that the first sheet flows to the shape of the mould and isconsolidated and bonded to the second sheet which is shaped by the mouldand at least a portion of which remains substantially unconsolidated andporous.

Preferably a substantial proportion of the fibres are between 7 and 50millimeters long and 13 microns or less in diameter. Conveniently, thefibres are also in the form of single discrete glass fibres. Such fibresare usually provided bonded together into chopped strand bundles andthese must be broken down into single fibres before the sheet is formed.

Where the fibres are required to confer structural strength in thelaminate, the discrete glass fibres should not be shorter than about 7millimeters or of a diameter greater than 13 microns, since such fibreswhich are longer do not adequately reinforce the plastics matrix andfibres which are of greater diameter do not so efficiently reinforce thematrix. Single fibres of other materials having a reinforcementefficiency at least as high as glass fibres may, alternatively, be used.

A high modulus of elasticity is to be taken as meaning a modulus ofelasticity substantially higher than that of the sheet. Fibres fallinginto this category include glass, carbon and ceramic fibres and fibressuch as the aramid fibres sold under the trade names Kevlar and Nomexand will generally include any fibre having a modulus higher than 10,000Mega Pascals.

In order to achieve the result set forth above the method may includeusing a second sheet of material which has been expanded as described inEuropean Patent Publication No. 0 148 763 above referred to, or in whichthe proportion of fibres is above that which is possible to achieve fullconsolidation.

This therefore makes use of the fact that in any rigid fibre (forexample glass)/polymer combination there is, due to the packing densityof the fibres, a critical fibre concentration above which fullconsolidation or densification of the structure is impossible undernormal conditions of pressing and moulding.

The method may include investing the porous side portion with athermosetting or thermoplastic plastics material as proposed in EuropeanPatent Application No. 85300035.4 (European Publication No. 0 152 994)the subject matter disclosed in that Application being incorporated byreference herein.

If desired the thermosetting or thermoplastic plastics material can beinvested in the mould.

Thus the thermosetting plastics material can be placed in the mould in aliquid state before investing the porous side. If it is to be investedwith a thermoplastics material this can alternatively be provided in theform of a third sheet prior to investment.

The thermoplastic materials may for example be of polyethylene,polypropylene, polystyrene, acrylonitrile-butadienestyrene,polyethyleneterephthalate polybutyleneterephthalate orpolyvinylchloride, both plasticised or unplasticised, or an alloy orblends of these materials with each other or other polyamide materials.Other suitable thermoplastics include polyphenylene ether orpolycarbonates or polyestercarbonates or thermoplastic polyesters orpolyetherimides or acrylonitrile--butylacrylate--styrene polymers oramorphous nylon or polyarylene ether ketone or alloys or blends of thesematerials with each other or other polymeric materials.

The fibre content of the first sheet is preferably less than 30% and thesecond sheet more than 60%.

With a glass fibre content of more than 60%, that is a material having acritical fibre concentration above which full consolidation anddensification of the structure is usually impossible under normalconditions of pressure and moulding, it is difficult to flow mould intointricate shapes but by combining such material with an easily flowmouldable material such as that set forth above, the desired effect canbe achieved.

The invention can be performed in various ways and various methods forproducing a glass fibre reinforced thermoplastics article and articlesmade by the methods will now be described by way of example and withreference to the accompanying drawings in which :

FIG. 1 is a diagrammatic view of a mould with a laminate material in itready for mould; and,

FIG. 2 is a diagrammatic cross-section showing an article produced bythe mould shown in FIG. 1.

As shown in FIG. 1 the fibre reinforced thermoplastics material to bemoulded comprises an upper sheet 1 of a fibre reinforced thermoplasticsmaterial which has glass fibres of a length of 13 mm and a diameter of11 μm in a polypropylene matrix. The content of glass fibres isapproximately 25%. This sheet is laminated on top of a second sheet 2which is of similar content to sheet 1 but which has a much higher glassfibre content, of approximately 80%. Due to the packing density of thefibres it will be impossible, under normal conditions of pressing andmoulding, to achieve a consolidated structure with this second sheet.

The mould in which the sheets are to be shaped into an article has anupper tool 3 provided with shaped indentations 4 and a lower tool 5having a cavity 6.

The laminated sheets, which may have been preheated to a predeterminedtemperature are placed in the mould which is closed to a fixed stop. Thehigh glass content material in sheet 2 conforms to the cavity 6 of thelower tool 5 whilst the low glass content material in sheet 1 is forcedto flow into the indentations 4 and take up the shape of the upper tool3. There is some intermingling of the materials at the boundary layerwhich ensures adequate bonding.

After cooling and deflashing, an article as shown in FIG. 2 is produced.The article, indicated by reference numeral 7, has an upper portion 8,formed from the sheet 1, of polypropylene which has become fullyconsolidated in which the uniformly dispersed glass fibre is indicatedby reference numeral 9. The lower portion of the article, which isformed from the sheet 2 is indicated by reference numeral 10, thisportion being porous and unconsolidated.

The article can be used in various ways, for example, it can be bondedto another article by utilizing the porous structure as a key for anadhesive or molten thermoplastic which unifies the two articles.Alternatively it can be made into a unitary article itself by fillingthe porous structure. In order to do this a thermosetting resin can bepoured or injected into the lower tool 5 (in this particularconfiguration) and the mould closed again so that the resin is forcedinto the porous, absorbent layer 10. After the curing the article isremoved and possesses the attributes of a fully reinforced mouldedthermoplastic on the upper side and a reinforced thermoset resin on theother. Additionally, sink marks are obscured, even before theimpregnation of the porous layer with a liquid resin.

This construction enables fast forming of flow moulded detail on oneside of the article combined with a smooth thermoset layer on the otherside. Thus, the article can have a good finish and have one surfacecapable of withstanding high temperature and with sufficient detail onthe other surface for stiffening or other requirements.

Table 1 specifies the theoretical and measured void contents ofunconsolidated material having a glass content above the critical levelat which consolidation can be achieved. The void content of thematerial, which subsequently renders it capable of impregnation was bothcalculated theoretically and determined by an oil absorption test. Itwill be seen that there was good agreement between the two modes ofevaluation.

                  TABLE 1                                                         ______________________________________                                        Void content of unconsolidated glass fibre/particulate                        thermoplastic composite sheet materials above the                             critical glass content at which consolidation can be                          achieved                                                                      Glass Content of fibres                                                       12 mm long 11 microns                                                         diameter       60%     70%     80%    90%                                     ______________________________________                                        Grammage (g/m.sup.2)                                                                         1114    1090    1099   1103                                    Bulk (cc/g)    0.88    1.12    1.39   1.66                                    *Theoretical Bulk (cc/g)                                                                     0.67    0.60    0.53   0.46                                    **Theoretical Void                                                                           24      46      62     72                                      Content (%)                                                                   +Oil Absorption (g/m.sup.2)                                                                  242.3   582.3   1075.8 1751.0                                  ++Void Content (%)                                                                           26      54      67     79                                      ______________________________________                                         *Density of glass fibre content  2.55 g/cc Density of thermoplastic           (polypropylene) content  0.91 g/cc                                            **Theoretical void Content based on measured sheet thickness and              theoretical bulk values                                                       +Density of oil used was 0.9 g/cc                                             ++Void Content based on volume of oil pick up                            

Table 2 sets out 8 examples of laminates formed from high and low glasscontent starting materials as specified in Note 1 of the Table. It willbe seen from the oil absorption test carried out on the side of eachlaminate formed from the high glass content component that the oilabsorption (and therefore the void content) values were substantiallyconsistent with the purpose for which the laminate is to be used.

                                      TABLE 2                                     __________________________________________________________________________    Lamination of High and Low Glass Content Materials                                                                   Laminate formed from High and Low                                             Glass                                                                         Content Material                       Glass Content    High Glass Content Material                                                                     Final           High Glass Content                                                            Lay                        Material                     Oil   Press                                                                             Total Total Estimated                                                                           Oil                       Substance                                                                           Thickness                                                                           Substance                                                                           Thickness                                                                           Absorption                                                                          Gap Substance                                                                           Thickness                                                                           Thickness                                                                           Absorption           Example                                                                            g/m.sup.2                                                                           (mm)  g/m.sup.2                                                                           (mm)  (g/m.sup.2)                                                                         (mm)                                                                              g/m.sup.2                                                                           (mm)  (mm)  (g/m.sup.2)          __________________________________________________________________________    1    2090  2.05  2040  2.82  1517  3.3 3668  3.25  1.30  715                  2    2090  2.05  1090  1.46  1076  3   3040  2.91  1.10  681                  3    2090  2.05  1090  1.46  1076  2   2440  2.09  0.56  182                  4    4180  4.10  1090  1.46  1076  3   3239  2.92  0.67  354                  5    4180  4.10   493  0.72   567  3   3000  2.78  0.17  233                  6    2090  2.05   493  0.72   567  2   2134  1.92  0.29  152                  7    2090  2.05   248  0.43   276  2   2026  1.87  0.19   79                  8    4180  4.10   248  0.43   276  2   2081  1.90  0.16   47                  __________________________________________________________________________     Notes:                                                                        1. Low Glass Content Materials = 25% glass fibre 12 mm long 11 microns        diameter, 75% Polypropylene powder by weight. High Glass Content Material     = 80% glass fibre 12 mm long, 11 microns diameter, 20% Polypropylene          powder by weight.                                                             2. The estimated thickness of layer derived from High Glass Content           Material was made from photographs of cross sections.                         3. The press temperature was set to 100° C. to facilitate flow         moulding flow moulding of the materials when pressure was applied.       

EXAMPLE 9

Unconsolidated samples of 70% glass fibre 12 millimeters long, 11microns diameter/30% polypropylene powder material having a substance of1000 g/m² and 25% glass fibre 12 millimeters long, 11 micronsdiameter/75% polypropylene powder material having a substance of 2000g/m² were cut to 22 cms diameter, which was the effective diameter ofthe mould. The samples were oven heated at 200° C. for 7 minutes andplaced in a press mould at a temperature of 100° C. with the 25% glasscontent material uppermost. On closing the press to a 3 mm final gap, adish was formed having an absorbent lower surface and moulded ribs andbosses in the upper surface. The press was raised and cooled to atemperature of 50° C., 40 g of a thermosetting resin (sold under theTrade name Modar 824 LT by ICI Ltd.) were poured into the mould and thepress reclosed, thus forcing the resin into the absorbent layer, withexcess resin being forced out of the tool. After curing, the dish wasweighed and calculated to have picked up 18 grams of resin, whichimparted a smooth, glossy finish to the lower surface.

EXAMPLE 10

Example 9 was repeated with unconsolidated sheet material samples of 80%glass fibre 12 millimeters long 11 microns diameter/20% polypropylenepowder having a substance of 1000 g/m² and 25% glass fibre 12millimeters long 11 microns diameter/75% polypropylene powder materialhaving a substance of 2000 g/m². A 24 g resin pick up resulted, with thelower surface having a finish similar to that of Example 9.

EXAMPLE 11

The following samples were prepared.

Two discs 23 centimeters in diameter and having a substance of 2000grams per square meter of unconsolidated permeable sheet like materialcomprising 25% glass fibres 12 millimeters long and 11 microns indiameter and 75% polypropylene powder, bonded together.

One disc 21 centimeters in diameter and having a substance of 500 gramsper square meter of unconsolidated permeable sheet like materialcomprising 80% glass fibres 12 millimeters long and 11 microns indiameter, and 20% polypropylene powder, bonded together.

One disc 17 centimeters in diameter of polycarbonate film 1 millimeterthick, sold under the trade name LEXAN by General Electric Co., whichacts as a third sheet.

The samples were heated in an oven at 205° C. for seven and a halfminutes and then placed together, in the order listed, in a press mouldat a temperature of 100° C. The press was then closed and a pressure of2000 lbs per square inch applied for one minute. The resulting laminatedmoulding proved to be substantially bonded.

EXAMPLE 12

The procedure of Example 11 was repeated, but with a polycarbonate filmsample 21 centimeters in diameter which replaced the disc of 17centimeters in diameter in Example 11. The resulting laminated mouldingwas found to have bonded well with the polycarbonate film partiallyenveloping the side edges of the moulding without creases having beenformed.

EXAMPLE 13

Samples were prepared as in Example 11, and the same procedure followedexcept that the samples comprised of polycarbonate were heatedseparately at 250° C. for four minutes and the bottom platten of thepress mould (in contact with the polycarbonate film) was maintained at140° C. during moulding. The procedure was then repeated twice withunconsolidated permeable samples comprising polycarbonate had glassfibre contents of 70% and 60% respectively.

The resulting laminated mouldings in all three cases were found to havebonded together with, and exhibited a greater resistance to forceddelamination, than the mouldings of Examples 11 and 12.

EXAMPLE 14

38 centimeter square samples were prepared from the following materials.

Unconsolidated permeable sheet like material having a substance of 2000grams per square meter and comprising 25% glass fibres 12 millimeterslong and 11 microns in diameter and 75% polypropylene powder, bondedtogether.

Unconsolidated permeable sheet like material having a substance of 500grams per square meter and comprising 80% glass fibres 12 millimeterslong and 11 microns in diameter and 20% polypropylene powder, bondedtogether.

Polycarbonate film sold under the trade name LEXAN by General ElectricCo. in thicknesses of 1 millimeter, 0.5 millimeter and 0.25 millimeter.

The two unconsolidated samples were heated to 205° C. for seven and ahalf minutes in an oven, and the 1 millimeter thick polycarbonate filmto 250° C. for four minutes. The samples were then placed in a platenpress at a temperature of 100° C. in the sequence listed and a pressureof 2000 lbs per square inch applied for 1 minute.

The foregoing procedure was then repeated on a second and a thirdoccasion in which the 0.5 millimeter and 0.25 millimeter polycarbonatefilms respectively were substituted for the 1 millimeter film.

A good bond between the three components of each of the resultinglaminates was found to have been formed.

EXAMPLE 15

The three laminates produced in Example 14 were cut to a diameter of 22centimeters. A 15 centimeter diameter piece of unconsolidated materialof the kind from which the first sample was cut in Example 14 was thenlaid on each of the circular laminates.

The resulting assemblies were in sequence heated to 205° C. for sevenand a half minutes and subjected to press moulding in a mould heated to120° C. at 2000 pounds per square inch for 1 minute.

The three resulting mouldings were found to be well formed and wellbonded.

EXAMPLE 16

Samples were first prepared for moulding as an in Example 9. The firstsample, comprising 70% glass fibre 12 millimeters long 11 micronsdiameter and 30% polypropylene was then oven heated to 200° C. for sevenminutes and then placed in the same press mould as had been used inExample 9. On closure of the mould, the structure of the sample wascompacted so that the molten polypropylene wetted out the surfaces ofthe glass fibres. As the press was raised, the resilience of the glassfibres caused the wetted out fibrous structure substantially to reassumeits porous configuration prior to pressing.

After the first sample had sufficiently cooled to be handled, it wasremoved from the mould and allowed to cool fully. 15 g of thermosettingresin sold under the trade name Modar 824 LT by ICI Ltd. were thenpoured into the mould and the first sample then returned to the mould.The press was then closed so that the thermosetting resin invested thepores in the lower surface of the sample. After curing, this produced adish-like structure having a smooth and glossy lower surface, and atangled open fibrous upper surface. The structure thus produced can beremoved from the mould for storage and integral moulding, at a laterdate, with a second fibre reinforced sheet comprising a substantiallyhigher proportion of thermoplastic, or it may be integrally moulded withsuch a sheet immediately, as described below.

Whilst the thermosetting resin was curing in the mould, the secondsample comprising 25% glass fibre 12 millimeters long 11 micronsdiameter and 75% polypropylene was heated to 200° C. for seven minutesand placed in the mould on top of the previously moulded first sample.The press was then closed for a second time so as to cause the hotmaterial at the lower face of the second sample to integrate with thetangled fibrous upper surface of the first sample already in the mould.Because of the relatively high thermoplastic content of the secondsample, it also flow moulded without difficulty to conform to theprofile of the upper portion of the mould.

We claim:
 1. A method of making a fibre reinforced plastics structurehaving a plurality of layers, including a first layer which is fullyconsolidated with uniformly dispersed fibres and a second layer which isformed as an absorbent matrix from a first sheet of fibre reinforcedthermoplastics material which will consolidate when cooled aftersubjection to pressure at a temperature above the melt temperature ofthe thermoplastics material of said first sheet of a second sheet offibre reinforced thermoplastics material which will remain porous whencooled after subjection to pressure at a temperature above the melttemperature of the thermoplastics material of the second sheet,comprising the steps of:(i) heating said first and second sheets to atemperature above that at which the thermoplastics materials of both ofsaid sheets are caused to melt, and (ii) subjecting said first andsecond sheets, whilst in surface contact, to pressure in a mould;wherein said first and second sheets bond together in conformity withthe shape of the mould and form a laminated moulding having a fullyconsolidated first layer and a second layer at least a portion of whichremains porous.
 2. The method of making a fibre reinforced plasticsstructure, as recited in claim 1, including the steps of selecting amaterial for the first layer having a fibre density which is less than acritical fibre concentration, and selecting a material for the secondlayer having a fibre density which is greater than the critical fibreconcentration, where the critical fibre concentration represents a fibredensity above which full consolidation of the structure is impossibleunder normal conditions of pressing and moulding.
 3. A method accordingto claim 1 wherein the first sheet and the second sheet are laminated.4. A method as claimed in claim 1 in which a substantial portion of thefibres are between 7 and 50 millimeters long and 13 microns or less indiameter.
 5. A method as claimed in claim 4 in which the fibres are inthe form of single discrete fibres.
 6. A method as claimed in claim 1 inwhich the reinforcing fibres have a high modulus of elasticity.
 7. Amethod as claimed in claim 1 which includes using a second sheet ofmaterial which has been expanded, or in which the fibre content is abovethat which is possible to achieve full consolidation.
 8. A method asclaimed in claim 1 which includes investing the said porous portion ofsaid second sheet of fibre reinforced plastics material with athermosetting of thermoplastic plastics material before or afterlaminating with said first sheet of fibre reinforced plastics material.9. A method as claimed in claim 8 which includes investing the porousportion with thermosetting or thermoplastic plastics material in themould.
 10. A method as claimed in claim 9 in which the thermosettingplastics material is placed in the mould in a liquid state prior to saidlaminate.
 11. A method as claimed in claim 8 in which the said porousportion is invested with a thermoplastics material which is provided insheet form and integrated with said laminate in the mould.
 12. A methodas claimed in claim 11 in which the thermoplastics material sheet is oflarger overall dimensions than the porous portion of the second sheet offibre reinforced thermoplastics material whereby part of the third sheetis caused to integrate with at least part of one side edge of the secondsheet.
 13. A method as claimed in claim 10 which includes moulding thesecond sheet under pressure, investing it with the thermosetting orthermoplastics material and then laminating said second invested sheetwith said first sheet.
 14. A method as claimed in claim 13 whichincludes placing the second sheet in the mould, applying pressure,releasing said pressure to allow said sheet to resume its porousconfiguration prior to pressing, placing the thermosetting orthermoplastic plastics material in the mould, applying pressure toinvest said second sheet with the thermosetting or thermoplasticplastics material and subsequently laminating said invested second sheetto said first sheet.
 15. A method as claimed in claim 1 which includespreheating the first and second layers before placing them in the mould.16. A method as claimed in claim 1 in which the fibres are glass fibres.17. A method as claimed in claim 16 in which the glass fibre content inthe first sheet is less than 30%.
 18. A method as claimed in claim 16 inwhich the glass fibre content in the second sheet is more than 60%. 19.A method as claimed in claim 1 in which the thermoplastics materials ispolyethylene, polypropylene, polystyrene, acrilonitrystyrenebutadiene,polyethyleneterephthalate, polybutyleneterephthalate orpolyvinylchloride, both plasticised or unplasticised, or an alloy andblends of these materials with each other or after polymeric materials.20. A method as claimed in claim 1 in which the thermoplastics materialsare polyphenylene ether or polycarbonates of polyestercarbonates orthermoplastic polyesters or polyetherimides oracrylonitrile--butylacrylate--styrene polymers or amorphous nylon orpolyarylene ether ketone or alloys or blends of these materials witheach other or other polymeric materials.